This disclosure relates to the casting of thin steel strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls to dam the two ends of the casting pool against outflow. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668; 5,277,243; and 5,934,359.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the casting of thin steel strip.
When casting steel strip in a twin roll caster, the strip leaves the nip at temperatures of the order of 1400° C., and if exposed to air, the strip suffers very rapid scaling due to oxidation of the strip at such temperatures.
It has therefore been proposed to shroud the newly cast strip within an enclosure containing a non-oxidizing atmosphere until its temperature has been reduced, typically to a temperature of the order of 1200° C. or less to reduce scale formation. One such proposal is described in U.S. Pat. No. 5,762,126 according to which the cast strip is passed through a sealed enclosure in which oxygen levels are reduced by initial oxidizing of the strip passing through the enclosure. Thereafter the oxygen content in the sealed enclosure is maintained at less than the surrounding atmosphere by continuing oxidizing of the strip passing through the enclosure and controlling the thickness of the scale on the strip emerging from the enclosure. The emerging strip may be reduced in thickness in an in-line rolling mill and then generally subjected to forced cooling, for example by water sprays, and the cooled strip is then coiled in a conventional coiler.
As more fully described in U.S. Pat. No. 6,585,030 and International Application PCT/AU01/01215, steel strip can be produced from molten steel of a given composition with any of a wide range of microstructures, and in turn a wide range of yield strengths, by continuously casting the strip and thereafter selectively cooling the strip to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C. It is understood that the transformation range is within the range between 850° C. and 400° C. and not that entire temperature range. The precise austenite to ferrite transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
Specifically, from work carried out on plain carbon steel, including low carbon steel that has been silicon/manganese killed or aluminum killed, it has been determined that selecting cooling rates in the range of 0.010° C./sec to greater than 100° C./sec, to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C., can produce steel strip that has yield strengths that range from 200 MPa to 700 MPa or greater. By selection of an appropriate cooling rate, it is possible to produce a microstructure which governs the yield strength selected from a group that includes microstructures that are (1) predominantly polygonal ferrite; (2) a mixture of polygonal ferrite and low temperature transformation products and (3) predominantly low temperature transformation products. The term “low temperature transformation products” includes Widmanstatten ferrite, acicular ferrite, bainite and martensite.
This development enables production of thin steel strip from molten steel of a given chemistry to meet differing customer-specified yield strength properties by varying the conditions under which the as-cast strip is cooled through the austenite to ferrite transformation range.
As described in U.S. Pat. No. 6,581,672, it is also possible to change other process parameters in the strip casting process to produce strip meeting varying customer-specified properties from a given strip casting line.
By the present disclosure, the thickness of the as-cast strip is controlled by changing the depth of the casting pool. This enables the casting rolls to be operated at a generally constant heat flux, which permits increased throughput without generating excessive wear temperatures at the casting surfaces, while varying the strip thickness. Accordingly, a single-roll profile may be used for casting rolls with a substantially constant throughput to produce a broad range of different cast strip thicknesses. Also, a constant as-cast microstructure can be maintained in the cast strip, which can consistently and predictably be modified and controlled by the subsequent cooling regime to produce strip having customer-specified properties. Further, increased flexibility in varying the thickness of the as-cast strip is provided that enables the subsequent reduction in the in-line rolling mill to be selected for desired strip thickness.
Specifically, described is a method of casting cast steel strip from a casting pool of molten steel using the casting surfaces of a twin roll caster to produce strip of differing thicknesses in the as-cast condition, comprising:                (a) determining for each desired thicknesses of the as-cast strip, a target casting speed which will avoid over-heating of the casting roll surfaces;        (b) determining from each target casting speed a target casting pool depth to produce a cast strip of the desired thickness when the twin roll caster is operated at the target casting speed; and        (c) operating the caster to cast strip based on the determined target casting speed and the determined target depth to produce cast strip generally of the desired thickness.        
The method may be performed with a single or twin-roll caster. The as-cast strip may have differing thicknesses, which may be customer-specified, or may be reduced, as by for example in-line rolling, to a desired customer-specified thickness.
In determining the target casting speed and the target casting pool depth, predetermined characteristics of the casting rolls of the roll casters such as the diameter of the casting rolls and heat flux rate through the casting surfaces may be factors to be considered. The casting rolls may include copper or copper alloy sleeves defining the casting surfaces of the rolls. In this case, the casting roll characteristics may include the diameter of the rolls and the thickness of the sleeves, which affect the relation between the casting speed and the casting surface temperature for a particular heat flux.
If these physical characteristics of the casting rolls remain essentially the same, then the caster can be operated at substantially the same production throughput rate, hence it is possible to calculate the target casting speed (u) for a given cast thickness, and then the target casting pool depth is varied to control the as-cast thickness of the strip, i.e., the target casting pool depth is decreased to decrease the as-cast thickness of the strip.
The casting pool depth is measured from the nip of the casting roll, where the strip departs from the casting surfaces of the casting rolls, vertically to the level of the casting pool. The target pool depth may be determined from the target casting speed in accordance with the following equation:
      R    ⁢                  ⁢                  sin                  -          1                    ⁡              (                  h          R                )              =      u    *                  d        2                    k        2                            where, h=pool depth (mm),        R=casting roll radius (mm),        d=half strip thickness (mm),        k=roll k-factor (mm/min0.5),        u=casting speed (mm/min).        
The roll k-factor is determined empirically by determining solidification rates in accordance with the formula:d=k√{square root over (t)}where d is the half strip thickness, and t is solidification time.
We have found that the selected depth of the casting pool may be monitored and controlled using image sensors such as cameras. Further, the flow of molten metal in the metal delivery system may be monitored and used to control the selected casting pool depth.
Also disclosed is a method of continuously casting metal strip comprising:                assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which thin cast strip can be cast, and a metal supply system capable of delivering molten metal above the nip;        forming a casting pool of molten metal supported on the casting surfaces above the nip to form a casting area;        sensing images of the casting pool in a plurality of locations in the casting area indicative of the casting pool depth in the plurality of locations;        displaying the sensed images to an operator; and        controlling a flow of molten metal from the metal supply system into the casting pool responsive to the sensed images indicative of the casting pool depth.        
The method of continuously casting metal strip may further include producing separate electrical signals corresponding to the sensed images indicative of the casting pool depth in each of the plurality of locations, receiving the separate electrical signals indicative of the casting pool depth in the plurality of locations, and controlling a flow of molten metal from the metal supply system into the casting pool responsive to one or more of the electrical signals. The electrical signals may be processed to determine the casting pool depth in each of the plurality of locations and the casting pool depth displayed to the operator.
One or a combination of separate electrical signals indicative of desired sensed images of the casting pool depth in the desired locations may be selected for providing a determined casting pool depth, and the flow of molten metal from the metal supply system into the casting pool controlled responsive to the determined casting pool depth. The method may include averaging the casting pool depths from the selected electrical signals for providing the determined casting pool depth.
The method of continuously casting metal strip may comprise in addition the steps of:                determining a target casting speed and a target casting pool depth to produce a cast strip of desired thickness when casting at the target casting speed;        determining the difference between the determined casting pool depth and the target casting pool depth; and        controlling a flow of molten metal from the metal supply system into the casting pool responsive to the difference between the determined casting pool depth and the target casting pool depth.        
The target pool depth may be determined in accordance with the following equation:
  h  =      R    ⁢                  ⁢          sin      ⁡              [                              u            R                    *                                    d              2                                      k              2                                      ]                            where h=pool depth (mm), R=casting roll radius (mm), d=half strip thickness (mm), k=roll k-factor (mm/min0.5), u=casting speed (mm/min), and k=d/√{square root over (t)} where d is the half strip thickness and t is solidification time.        
The metal supply system may include a tundish capable of delivering molten metal through a distributor to a delivery nozzle, so that the step of controlling the flow of molten metal from the metal supply system into the casting pool is performed by controlling the flow of molten metal from the tundish to the distributor. The weight of the molten metal in the distributor may be sensed, producing electrical signals indicative of the weight of the molten metal in the distributor, and the flow of molten metal from the tundish to the distributor controlled responsive to the electrical signals indicative of the weight of molten metal in the distributor.
A method is also disclosed of continuously casting metal strip may include:                sensing an image of the flow of molten metal from the metal supply system into the delivery nozzle in a plurality of locations in the casting area;        displaying the sensed images to the operator; and        controlling the flow of molten metal from the metal supply system into the delivery nozzle responsive to the sensed images of the flow of molten metal into the delivery nozzle.        
The method of continuously casting metal strip may further include producing electrical signals corresponding to the sensed images of the flow of molten metal into the delivery nozzle in each of the plurality of locations, receiving the electrical signals indicative of the flow of molten metal into the delivery nozzle in the plurality of locations, and controlling the flow of molten metal from the metal supply system into the delivery nozzle responsive to the electrical signals indicative of the flow of molten metal into the delivery nozzle. At least a portion of the metal supply system may be maintained responsive to the sensed images of the flow of molten metal into the delivery nozzle.
Sensing images may be performed by a plurality of digital or analog cameras operatively positioned in the casting area, and in one configuration may include sensing images of the casting pool in at least four locations in the casting area. In addition, at least one camera may be operatively positioned to sense images adjacent a side dam retaining the casting pool at an end of the casting rolls (in the area known as the triple point region). This sensing of images in the triple point region may be done by such cameras directly, or remotely by positioning fiber optic sensors in the triple point region. The triple point region is the interface between the side dam, the casting rolls, and the casting pool. Also, the sensing of images in the casting area may include providing a plurality of fiber optic sensors operatively positioned in the casting area.
In an alternate method of continuously casting metal strip, the steps include:                assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which thin cast strip can be cast, a tundish capable of delivering molten metal through a distributor to a delivery nozzle capable of delivering molten metal above the nip and forming a casting pool of molten metal supported on the casting surfaces above the nip in a casting area with side dams adjacent the ends of the nip to confine the casting pool;        sensing the weight of the molten metal in the distributor and producing electrical signals indicative of the weight of the molten metal in the distributor; and        controlling flow of molten metal from the tundish to the casting pool responsive to the electrical signals indicative of the weight of molten metal in the distributor.        